It is of primary importance for the operation of an elevator system that the elevator system operates faultlessly and above all safely. For this reason numerous different safety devices are in use in elevator systems. One of these is the so-called safety circuit. A safety circuit, which travels from one safety device of the elevator to another, is the most important part of the electrical safety system of an elevator. The circuit comprises chained contacts and switches such as the switches of a static circuit, the switches of the car and landing door, the switches of locks and the contactor of the elevator. If even one safety device disconnects the safety circuit, the elevator stops or it will not start moving. If e.g. the doors of the elevator car are open, the safety circuit is open and the elevator will not start moving before the doors are closed and the safety circuit in its entirety is closed.
An elevator that is already in use must be serviced and its condition checked under legal requirements to ensure its safe operation. In the inspection of the condition of the elevator operating tests are performed on it, i.e. the functioning of the safety devices and alarm devices are tested and it is checked that the elevator will not move before the car and landing doors are closed, and that they will not open before the elevator is at a floor. The safety circuit and its contacts and switches thus have an important role in the condition monitoring of the elevator. When inspecting the condition of an elevator different condition monitoring appliances can be used, the analyzers of which can utilize information about the current traveling in the safety circuit.
Inferences can thus be made about the condition of the elevator by examining the operation of the safety circuit of the elevator and its safety switches. On the basis of the strength of the current traveling in different points of the safety circuit it can be inferred which switches that belong to the safety circuit are open or closed at each moment in time, and whether the switches of the safety circuit operate according to the regulations set for them.
Depending on the states of the contacts of the safety circuit, i.e. whether each switch is open or closed, the current traveling in the safety circuit receives different levels of strength. During the start-up of the elevator samples are received of all the current levels of the safety circuit, which are classified. As a result of this a class center of current strength corresponding to each state of the safety circuit is ascertained. After start-up, during normal operation of the elevator, the state of the safety circuit can be determined by means of the current measured from the safety circuit, e.g. by seeking the class center most closely corresponding to the strength of the measured current.
The switches in the dynamic part of the safety circuit change their state all the time during normal operation of the elevator, in which case they wear and do not necessarily connect properly. Also dirt, grease, etc., that enters the switches may cause contact malfunctions. If even one of the contacts of a safety circuit does not connect, the elevator is not able to move or, if the elevator is moving, it stops. Finding a defective switch is awkward because all the switches are in series and they are situated on different sides of the elevator shaft far apart from each other. The malfunctioning may also be concealed, in which case one of the switches causes random switching interference.
Bad contacts of the switches can be indirectly monitored e.g. via failed starts or by re-openings of a fully-closed door. The use of monitoring of re-openings of the doors in determining contact malfunctions is based on the fact that the control system endeavors to make the safety circuit unbroken by re-opening a door. A traditionally awkward situation from the standpoint of the serviceman is if the elevator has stopped between floors in the middle of a run. If the source of the problem is the contacts of the door or of the lock of a landing, and there is no remote monitoring in the elevator, locating the fault can be awkward. In this case it is possible that the elevator starts to run e.g. after a restart (reset), in which case the fault remains concealed and causes a new fault situation after some time has passed. The location of the switches may also be awkward to access, in addition to which the switches are located in the elevator shaft a considerable distance from each other, especially in a high-rise building.
Thus by means of also modern remote monitoring it is possible to indirectly monitor contact malfunctions via re-openings and attempted starts. In this case, however, contact malfunction has progressed very far and they cause fault situations. In addition, re-openings and failed starts erode the transportation capacity of the elevator and impair ride comfort.